Gate valves are generally comprised of a valve body having a central axis aligned with inlet and outlet passages, and a space between the inlet and outlet passages in which a slide, or gate, may be moved perpendicular to the central axis to open and close the valve. In the closed position, the gate surfaces typically seal against sealing rings which surround the fluid passage through the valve body. Gate valves have been used to control the flow of a great variety of fluids. Often the fluid to be controlled by the gate valve is under pressure. In the petroleum industry, gate valves are used along piping at various locations, and in particular are used in piping referred to in the petroleum industry as a christmas tree, which surmounts a wellhead and is used to control flowline distribution of production fluids from a well for a producing zone or for a storage reservoir.
Actuators to open and close the gate valves may employ manual operators, diaphragm-type operators, and hydraulic operators. The actuator typically includes a bonnet assembly, which interconnects the operator body and the gate valve body, and a bonnet stem which is movable with the gate via the operator.
Surface safety systems are used to control the actuator operator to close the gate valve in a fail-close operation and assure that the source of the fluids (the well or storage reservoir) is isolated from a flowline either if a flowline ruptures or otherwise experiences a loss in pressure (low pressure shutoff), or if the flowline experiences a higher pressure than the flowline is rated to handle (high pressure shutoff). The gate valve in such a system may be called a surface safety valve. A second safety valve may be located in the well production tubing several hundred feet below the ground surface, for instance, about 600 feet below the wellhead. Subsurface safety valves usually are installed for offshore wells, but sabotage destruction of surface safety systems has raised interest in use of subsurface valves for onshore application. The subsurface safety valve typically is a flapper or ball type valve which may be carried in a tubing connection or may be installable and set in place by wireline.
In a type of safety valve system, fluids at well pressure from the well under production are employed in hydraulic circuits to operate the actuator to open the surface and subsurface safety valves if pressures are within predetermined limits. These production fluids are also fed to high and low pressure pilots which control shut down of the system. When a low or high pressure condition exists, the respective pilot will trip, venting line pressure to the atmosphere and causing a check valve to block supply of line pressure to the actuator. The actuator then vents off pressure to the atmosphere through the pilot exhaust to close the valve. This position is maintained until the check valve is manually reset after the cause of the out-of-limits pressure experience is determined and corrected.
A problem with this prior approach is that the venting of petroleum gases and /or fluids into the atmospheric in this typical system is environmentally unsatisfactory, and can be dangerous and potentially lethal where the produced fluid contains hydrogen sulfide or carbon monoxide, especially if an oilfield worker is adjacent the wellhead when these poisonous gases at well pressures are automatically vented.
In high pressure wells, fluids pass through the open subsurface safety valve seals at high rates frictionally creating high temperatures that are destructive to seal life. A problem with the safety valve systems of the prior art, especially in high pressure wells, grows out of deteriorating seals in the subsurface safety valve. When the seals of the subsurface safety valves deteriorate, well pressure can leak past the seals of the closed subsurface safety valves into the reservoir, which may be objectionable to the well operator, and in any case, reduces the control line pressure above the closed valve and can allow the subsurface valve to re-open. This then allows production fluids to enter and flow through the valve control system lines. If those lines vent to the atmosphere, then the production fluids can escape from the well into the environment. The entry and passage of the well fluids through the safety control lines may also damage the surface safety system.
Damage to the subsurface valve requires removal of the valve apparatus from the subsurface location to a surface location where it can be repaired. In a tubing carried subsurface safety valve, this means a removal of tubing, at great expense and loss of revenues from well downtime. For a wireline set subsurface safety valve, retrieval is less onerous, but still requires a wireline rig to pull the valve apparatus, still at unwelcomed expense and well downtime.
It is an object of this invention to provide a completely self contained hydraulically operated surface safety valve actuator system which avoids contamination of the environment and eliminates the risk of harm to oil field workers from sudden release of hydrogen sulfide and/or other poisonous gases at the wellhead.
It is an object of this invention to provide a completely self contained hydraulically operated surface safety valve actuator system which does not make use of production line pressure to operate the actuator and which does not vent production fluids into the atmosphere.
It is a further object of this invention to provide a system for closing both surface and subsurface safety valves in a manner that prevents escape of production fluids to the atmosphere and environment, and which also eliminates injection of line control fluids into the reservoir from which the well is being produced.
These and other objects, benefits and advantages of the system of this invention will be apparent from the following description of the invention.